The present invention relates to programmable integrated circuits employing semiconductor memories in the form of insulated-gate storage transistors whose threshold voltages can be changed by injection of charge carriers into or out of a storage medium of the individual insulated-gate field-effect transistors and more particularly to an arrangement to present a faulty programming of individual storage transistors by an unintended displacement of the threshold voltage of individual storage transistors.
The present invention is based on the recognition that at least some of these faulty programmings are due to the fact that during the unintended or intended disconnection of the power supply, at the moment of the loss of this voltage, it is possible for unintended programming pulses to be triggered which destroy the information contained in the memory. This occurs above all when, as is the ususal case with non-volatile memories, the voltage supply is provided by several sources.
The invention, therefore, is concerned with the problem of preventing as far as possible such unintentionally occuring programming pulses in an integrated circuit comprising non-volatile programmable semiconductor memories in the form of insulated-gate field-effect transistors whose threshold voltages, at least during the writing operation, can be changed with the aid of a momentarily applied writing pulse.
A review of the involved non-volatile programmable semiconductor memories can be found on pages 1039 to 1059 of "Proceedings of the IEEE", Vol. 64, No. 7 (July 1976). With these semiconductor memories, at least the writing is carried out by means of an electric pulse with the aid of which charge carriers tunnel through a sufficiently dimensioned insulating layer to a storage medium in the direct proximity of the channel region of an insulated-gate field-effect transistor, and are trapped in the storage medium. At present, integrated circuits employing nonvolatile programmable semiconductor memories are commercially available in the form of MNOS transistors. Relative thereto, reference is made to the printed publication "Siemens Forschungs- und Entwicklungsberichte", Vol. 4 (1975) No. 4, pp. 213 to 219. A further practical application of the invention is with insulated-gate field-effect transistors with storage media in the form of gate electrodes with a floating potential as known, for example, from "IEEE Transactions on Consumer Electronics", Vol. CE-26 (February 1980), pp 20 to 25.
The range of practical application of the invention also extends to integrated circuits employing semiconductor memories which are written electrically but can also be erased by ultraviolet radiation, as is known, for example, from "Electronics" of Mar. 13, 1980, pp. 115 to 121.
Integrated circuits to which the invention relates, at least for writing in the storage transistors, comprises a programming logic, mostly in a special substrate zone for writing optionally into each of the storage transistors a logic "one" or logic "zero" defined by the threshold voltage. For this purpose the gate electrodes of the storage transistors are connected wordwise to a word line.